This invention is concerned with a method for optically measuring a distance about three dimensional objects.
Conventional methods of this kind have very precise accuracy such as a method using light interference, a method using holography, a method using moire image interference fringes and others. But these methods are available for laboratory use, but not suitable for on-line use in a plant.
Optical measuring method of triangulation not possible or available for on-line use in these optical measuring methods. The method of the present invention is based on a triangular measuring method, with the theory, being described first by referring to FIG. 1.
A laser beam a is projected on a base line 1 toward a mirror 2 from which it is reflected as beam b which is irradiated on an object 3 making a spot P on the object. Spot P is detected by a photo-detector 4 by way of optical axis 7 nestled between a pair of slits 5 and 6. The distance M between spot P and base line 1 can be calculated if the distance l between the intersecting point Q.sub.1 of detecting optical axis 7 to base line 1 and the reflecting point Q.sub.2 of laser beam a at mirror 2, are known also if the intersecting angle .theta..sub.1 of detecting optical axis 7 to base line 1, and the intersecting angle .theta..sub.2 of reflecting light line 6 to laser beam a, are known. In a practical method, mirror 2 is fixed so that angle .theta..sub.2 has a constant value. By moving the combined photo-detector 4 and slits 5 and 6 parallel to base line 1, keeping angle .theta..sub.1 constant, the distance l will be determined. From the moving amount of movement of combined slits and photo-detector 4, until spot P is detected, whereby the distance M is calculated. In another method, the combined photo-detector 4 and slits 5 and 6 are relatively fixed to keep angle .theta..sub.1 constant, while the mirror 2 rotates to sweep the reflected light beam b on object 3. By this method, the distance M can be calculated using angle .theta..sub.1, angle .theta..sub.2 which is equal to 2.theta..sub.3, (.theta..sub.3 is the rotating angle of mirror 2 referred to base line 1 when spot P is detected by photo-detector 4), and distance l=d.sub.2 - d.sub.1 sec .theta..sub.3, with - d.sub.3 tan .theta..sub.3, d.sub.1 indicating the distance between the rotating axis of mirror 2 and the mirror surface. d.sub.2 indicates the distance between the rotating axis and intersecting point Q.sub.1, and d.sub.3 indicates the distance between the rotating axis and base line 1.
In this latter method, high accurate measurement of 2.mu.m unit requires very stable values in distances d.sub.1, d.sub.2 and d.sub.3 and in angle .theta..sub.1 high precise measurement of the rotating angle .theta..sub.3 of the mirror is also required. Distance d.sub.1 is rather stable but d.sub.2, d.sub.3 and .theta..sub.1 are strictly unstable due to thermal displacements. Particularly in measurement of the profile of the object by continuously moving the mirror and the photo-detector along the base line, it becomes extremely difficult to keep d.sub.3 and .theta..sub.1 constant. .theta..sub.1 and d.sub.3 should in fact be treated as variables. There is a similar problem in the former conventional method, in which distance l and angle .theta..sub.1 are also variables due to error in the photo-detector movement.